Toner binders used in toners for developing electrostatic images are required to satisfy two contradictory requirements, namely they should have the ability to be fixed even at a low hot roller temperature (low temperature fixability) and the inability to fuse to the hot roller even at a high hot roller temperature (anti-hot offset property).
A number of proposals have been offered to improve the low temperature fixability and anti-hot offset property of polyester-based toner binders by blending two different polyesters differing in molecular weight distribution (e.g. Japanese-Kokai Publication 2001-265056). These use two polyesters differing in molecular weight distribution as a low viscosity polyester for meeting the low temperature fixability requirement and as a high viscosity polyester for meeting the anti-hot offset property requirement. In recent years, a higher and higher level of low temperature fixability has been demanded from the energy saving viewpoint and, from the viewpoint of miniaturization of copiers and like apparatus, a higher and higher level of anti-hot offset property has been demanded.